ORIGINAL PAPER Helix-like biopolymers can act as dampers of force for bacteria in flows Johan Zakrisson • Krister Wiklund • Ove Axner • Magnus Andersson Received: 7 February 2012 / Revised: 4 April 2012 / Accepted: 15 April 2012 Ó European Biophysical Societies’ Association 2012 Abstract Biopolymers are vital structures for many liv- ing organisms; for a variety of bacteria, adhesion polymers play a crucial role for the initiation of colonization. Some bacteria express, on their surface, attachment organelles (pili) that comprise subunits formed into stiff helix-like structures that possess unique biomechanical properties. These helix-like structures possess a high degree of flexi- bility that gives the biopolymers a unique extendibility. This has been considered beneficial for piliated bacteria adhering to host surfaces in the presence of a fluid flow. We show in this work that helix-like pili have the ability to act as efficient dampers of force that can, for a limited time, lower the load on the force-mediating adhesin-receptor bond on the tip of an individual pilus. The model presented is applied to bacteria adhering with a single pilus of either of the two most common types expressed by uropathogenic Escherichia coli, P or type 1 pili, subjected to realistic flows. The results indicate that for moderate flows (*25 mm/s) the force experienced by the adhesin-receptor interaction at the tip of the pilus can be reduced by a factor of *6 and *4, respectively. The uncoiling ability pro- vides a bacterium with a ‘‘go with the flow’’ possibility that acts as a damping. It is surmised that this can be an important factor for the initial part of the adhesion process, in particular in turbulent flows, and thereby be of use for bacteria in their striving to survive a natural defense such as fluid rinsing actions. Keywords Fimbriae  Pili  Uncoiling  Damping  Bacterial adhesion Introduction Biopolymers are large macromolecules produced by living organisms that are composed of repeating monomeric sub- units. Although they have well-defined primary and second structures, they frequently and spontaneously arrange into characteristic higher order (tertiary and sometimes even quaternary) structures that often have a decisive impact upon their biomechanical properties. They play a variety of roles in biological systems. For example, those that are expressed on the outer membrane of bacteria and convey an adhesin on their tip that binds to specific receptors on the surface of host cells, alternatively referred to as attachment organelles, fimbria, adhesive fibers, or pili, are assumed to have an important role in the initial attachment of bacteria to host tissue (Duncan et al. 2005). To adapt to different environments, presumably because of evolutionary selec- tion, various types of bacteria express organelles with dif- ferent types of higher order structures. Whereas some are shaped like an open coil (linear chain) (Hilleringmann et al. 2008), others have a higher complexity with a helix-like quaternary shape (Bullitt and Makowski 1995). This gives various types of organelles dissimilar biomechanical prop- erties (Axner et al. 2011; Castelain et al. 2009). The biomechanical properties of attachment organelles are often assessed by their force-extension response by the use of force spectroscopic techniques, e.g., optical tweezers Electronic supplementary material The online version of this article (doi:10.1007/s00249-012-0814-8) contains supplementary material, which is available to authorized users. J. Zakrisson  K. Wiklund  O. Axner  M. Andersson (&) Department of Physics, Umea ˚ University, 901 87 Umea ˚, Sweden e-mail: magnus.andersson@physics.umu.se J. Zakrisson  K. Wiklund  O. Axner  M. Andersson Umea ˚ Centre for Microbial Research (UCMR), Umea ˚ University, 901 87 Umea ˚, Sweden 123 Eur Biophys J DOI 10.1007/s00249-012-0814-8